The present invention relates to a suspension control apparatus which can be advantageously used as a suspension control apparatus for an automobile.
As a conventional suspension control apparatus, there can be mentioned an apparatus comprising a proportional solenoid valve having a spool for controlling an amount of a hydraulic fluid passing therethrough according to a position of the spool, the spool being adapted to be displaced according to an intensity of current applied to a solenoid; a variable damping force generating shock absorber provided between a car body and an axle to generate a damping force which varies depending on the intensity of the current applied to the solenoid, that is, depending on the position of the spool; and an acceleration sensor for detecting an acceleration of the car body in a vertical direction. In this apparatus, the intensity of current to be applied to the solenoid is determined, based on the value detected by the acceleration sensor, to thereby generate a desired damping force (for an extension stroke and a compression stroke). Thus, damping of vibration of a sprung mass is effected, thereby improving the ride quality.
In the above-mentioned apparatus, the current to be applied to the solenoid is obtained by superimposing a high-frequency oscillating current (dithering current, which is hereinafter referred to simply as "dither"), which is obtained on the basis of a PWM (pulse width modulation) signal, on a fundamental current having a desired intensity (a desired current having an intensity equal to an average intensity of the current to be applied to the solenoid), which is determined from the value detected by the acceleration sensor. Therefore, the spool oscillates slightly (dithers) about a predetermined position, so that displacement of the spool can be easily performed, thereby improving responsivity in controlling a damping force. The above-mentioned current to be applied to the solenoid, which includes a dither is obtained, for example, in a manner as mentioned below.
A switching device (transistor) is provided between the solenoid and a power source. The switching device is adapted to be switched on and off in response to the PWM signal. Due to a transient phenomenon, when the PWM signal is of a high level [for example, when a duty ratio of the PWM signal is 75% (that is, when the transistor is switched on during a period of 75% of a cycle of the PWM signal and switched off during a period of 25% of a cycle of the PWM signal)], the current intensity applied to the solenoid gradually increases. Subsequently, when the PWM signal is of a low level [for example, the duty ratio of the PWM signal is 25% (that is, the transistor is switched on during a period of 25% of a cycle of the PWM signal and switched off during a period of 75% of a cycle of the PWM signal)], the current intensity applied to the solenoid gradually decreases. Illustratively stated, as shown in FIG. 9, when the duty ratio of the PWM signal is as high as, for example, 75% (section A), a rate of increase in current intensity applied to the solenoid (when the transistor is switched on during a period of 75% of a cycle of the PWM signal) is larger than a rate of decrease in current intensity applied to the solenoid (when the transistor is switched off during a period of 25% of a cycle of the PWM signal). Consequently, in section A, the current intensity applied to the solenoid increases at each cycle of the PWM signal. (Hereinafter, for the sake of convenience, the duty ratio when the intensity of current applied to the solenoid increases at each cycle of the PWM signal is referred to simply as "increase duty ratio"). On the other hand, when the duty ratio of the PWM signal is as low as, for example, 25% (section B), a rate of decrease in current intensity applied to the solenoid (when the transistor is switched off during a period of 75% of a cycle of the PWM signal) is larger than a rate of increase in current intensity applied to the solenoid (when the transistor is switched on during a period of 25% of a cycle of the PWM signal). Consequently, in section B, the intensity of current applied to the solenoid decreases at each cycle of the PWM signal. (Hereinafter, for the sake of convenience, the duty ratio of the PWM signal when the current intensity applied to the solenoid decreases at each cycle of the PWM signal is referred to simply as "decrease duty ratio"). The duty ratio is switched between the increase duty ratio and the decrease duty ratio at each predetermined duty-ratio-switching cycle (1/2 of a cycle of a dither). Thus, the current to be applied to the solenoid is obtained in such a form that a dither having a predetermined amplitude and a cycle time of twice the duty-ratio-switching period is superimposed on the fundamental current (desired current). Hereinafter, for the sake of convenience, the amplitude of the dither is frequently referred to simply as "dither amplitude", and the duty ratio of the PWM signal is frequently referred to simply as "PWM duty ratio".
In the above-mentioned conventional suspension control apparatus, it is desired to maintain the dither amplitude at a predetermined level. However, when the voltage impressed on the solenoid changes due to a temperature change or the like, the dither amplitude also changes, so that the dither amplitude may fall outside of the predetermined range (optimum amplitude range). When the dither amplitude exceeds the upper limit of the optimum amplitude range, the proportional solenoid valve generates pronounced noise and vibration. When the dither amplitude falls below the lower limit of the optimum amplitude range, displacement of the spool is difficult to perform, so that an undesirable increase in hysteresis with respect to the damping force occurs, leading to poor responsivity in controlling the damping force.
The meaning of "hysteresis with respect to the damping force" is as follows. Hysteresis with respect to the damping force describes a phenomenon whereby to obtain an equal change in damping force when increasing or decreasing a damping force, different intensities of current are required to be applied to the solenoid. Therefore, in a graph showing a damping force-current intensity relationship, in which the abscissa indicates the intensity of current and the ordinate indicates the damping force, a closed curve (hysteresis loop) substantially in the form of a parallelogram is obtained. It is preferred that the size of the hysteresis loop (a difference of an intensity of current required for increasing a damping force to obtain a predetermined amount of change in damping force from an intensity of current required for decreasing a damping force to obtain the same predetermined amount of change in damping force) be suppressed to as low a level as possible.